Fault Surfaces in Earth Volumetric Studio

There are normally two reasons to create fault surfaces in EVS.  We either just want to display them, or we want to use them to create fault blocks as part of a more complex geologic model.  When we create fault blocks, we are using the surf_cut module and the fault surface is input to the right input port.  There are three main cases to consider with respect to the creation of fault surfaces:

1) Planar surfaces.  A planar surface is a surface which is flat.  It can be oriented in any way, but is has no curvature, bumps, etc.  This type of fault if very easy to create with the create_fault_surface module.  However, you can also cut any 3D model using the “cut” module to create a fault block.

2) Non-planar surfaces created in EVS.  There are two cases here.  Single-Valued and Multi-Valued surfaces

  • Single-Valued or simple surfaces can be created using krig_3d_geology, krig_2d or scat_to_tin using x-y-z points formatted as .GEO, .GMF or .APDV files.  This is the easiest way to create more complex surfaces which have only ONE Z coordinate for any X-Y location.
  • Multi-Valued surfaces are surfaces which which have TWO or more Z coordinates for any X-Y location. Some simple examples would be a sphere or a tube (pipe).
    • Some are easily created in EVS.  For examples it is easy to create tubes of constant or variable diameter with the tubes module, and these can be used to cut tunnels through geologic models.
    • However, in general the creation of multi-valued surfaces is a very difficult process since traditional estimation methods such as kriging, IDW, splining, TIN, etc. cannot be used.  In some cases it is possible to find a coordinate transformation where the surface would not be multi-valued.
For example, imagine a cross-section through a multi-valued fault surface below.  The red line shows where this surface is multi-valued.  If we rotate counterclockwise 90 degrees, this surface is no longer multi-valued and could be created using any of our traditional methods.  All we need to do is rotate the data, create the surface and then counter-rotate the resultant surface back.  We can rotate the data using transform_field, create the TIN (or krige, etc.) , and then rotate back to the original coordinates.  Be sure to use the same center of rotation in transform_field.
To confirm everything is correct, we check it against the original data.  Sometimes the angle needed is not 90 degrees, but since you don’t need to do the math yourself, it doesn’t matter.  In the case below, it would probably be better to use -75 vs. -90, since the “rotated new” left edge is nearly vertical.

However:  Sometimes, there is no set of rotations that will work, as in the case of a sphere. 

3) Multi-Valued surfaces created outside of EVS.  Though we don’t always provide ways to create every possible type of complex multi-valued surface, if you do have a complex multi-valued surface that was created in CAD software or by some other method, it can still be used in EVS as a fault cutting surface.  There are just a few simple requirements:

    • The surface can be composed of triangles or quadrilateral cells.  When these cells are defined, the order that the cell’s nodes define the cells determine the normal vector of the cell.  The normal vector defines the INSIDE and OUTSIDE of the surface.  It is important that all adjacent cells have consistent normal vectors.  Otherwise, it is like you are trying to cut with a net instead of a surface.  You can confirm you have consistent normals by setting “Face Culling” to “Remove Back Facing”  If your surface then appears to have holes in it, your normal vectors are not consistent.   This is still not the end of the world.  The tri_tool module provides a way to Fix Normals, but it can be tedious depending on the complexity of your surface.  Note: surfaces created by EVS will never have inconsistent normals.
    • Closed Surfaces: If the surfaces are closed, like a sphere, they can be anywhere and of any extent.  By this we mean:
      • They can be completely inside of the volume you are cutting, or
      • They can intersect the volume, or
      • They can be outside of the volume
    • Open Surfaces: If the surfaces are open, like a fault or excavation surface, it is important that the surface extend beyond the extent of the volume you are cutting.  This is necessary because it becomes ambiguous what should happen when we reach the edge of the surface if we are still inside or near the volume.